Patent document 1 discloses a conventional motor drive apparatus, which has a plurality of (for example, two) electric power supply sources such as inverters. When one of the inverters fails in this motor drive apparatus, supply of power from an abnormal inverter, which failed, to a plurality of winding sets of an electric motor is stopped and, instead, the power is supplied to the corresponding winding sets from the normal inverter other than the abnormal inverter. The motor is thus operated continuously by only the normal inverter, even when a part of the inverters fails.
Patent document 1: JP 2005-304119A
In the conventional apparatus according to patent document 1, the output of the abnormal inverter drops at the same time as the failure arises, when the supply of power to an abnormal power supply system is stopped at the time of detection of failure. As a result, the operation of the apparatus changes suddenly immediately after the failure. In case that the motor drive apparatus is applied to an electric power steering apparatus of a vehicle, for example, a sudden change in the output of the motor arises against a driver's normal steering operation.
If the loss of the output is compensated continuously by the operation of the other normal set to reduce the sudden change, the inverter in the normal power supply system overheats because of excessive load current. The motor drive apparatus is likely to be damaged by this heat. If the electric power capacity of the inverter is increased to reduce overheating, the inverter will have to be sized large and become costly. It is thus not possible to provide a motor drive apparatus, which is small in size and low in cost.